Bananas are Hard to Digest: So Said Your Great Grandmother

Posted on 16 Sep 2012 02:33 by EricT

Bananas are difficult to digest. This myth was being circulated in the early 1900's and probably earlier. The belief shows up in some health and nutrition books of the time, but whether the myth was inspired by the books, or the public belief was repeated by the authors without any investigation, is not immediately apparent.

It is a funny myth being that bananas are such a highly recommended staple of nutrition, and are perhaps rated higher than any other fruit. No matter, this myth was already being busted by nutrition experts in the mid 1930's and on into the 40's, although I'm sure it took a while before it was completely stamped out.1,2 Have you ever heard anyone say it?

You will often read or hear that unripe bananas are difficult to digest. Perhaps that is the origin of the myth and the 'unripe' part simply got discarded.

Well, we can figure it out if we look into banana history just a little bit. First, realize that the banana has always been the subject of rumors and even a little fear, since it was introduced into the Western market in the early 1900's.

Have you heard the one about spiders (snakes too) hitchhiking around the globe in banana crates, hidden in the bunches? Unfortunate people buy the bananas, and when they get home and decide to peel and eat one of the bananas, they are bitten by a spider that happens to be the most poisonous in the world, and then their finger rots off, or other gruesome things happen.

Just an example. But the myth of the hard-to-digest-banana we can certainly trace to the green banana. You see, there are many different varieties of bananas and not all of them are suitable for eating raw. Some of them are so starchy that they need to be cooked first, such as the familiar plantain. All these varieties are part of the genus Musa.

sliced fried plantains on a plate

The bananas we consume raw are generally referred to as sweet bananas. Well, if you are someone wanting to export a sweet banana from South America in the early 1900's, and you wait until it is ripe, and thus sweet, by the time you get it to the U.S., or anywhere else, it will be rotten. Not to mention that it is unwise to let culivated bananas ripen on the plant, as I reveal in 6 Surprising Facts About Bananas.

But when it is rock-hard and green, you might realize, it can be picked but will still slowly ripen over weeks, or even months, while you're shipping it. And for you, shipping means an actual ship, not a train ride or a truck haul.

To consumers of the time, however, this caused problems, adding to the natural distrust of a new food shipped from mysterious and dangerous foreign lands. First of all, some of the consumers got the banana while it was still green. Second, some people didn't think a fruit that had not ripened naturally, but instead had ripened while sitting in crates on a ship, could be trusted. They weren't really ripe. Basically, unripe bananas, or bananas that had been picked prematurely but had apparently ripened anyway..were sort of seen as the same thing. Hey, do you eat green unripe apples? No, they will give you a stomach ache. Everybody knows that. Well, don't eat these unripe bananas either.

The starch in the green banana was explained, by experts of the time, to be much like the starch in a potato, which is certainly very difficult for us to digest, if uncooked. The heat of cooking modifies the starches in such a way that makes them easier to digest, much like the natural ripening process changes the starches to simpler sugars.

many bananas

Bananas, and Possibly some Spiders and Snakes
Image by Daniel Sone

Other Banana Myths

One health related book written by, ahem, doctors, in 1895, entitled The Relation of Food to Health and Premature Death actually said that green bananas could bring on attacks of deadly cholera! They went on to claim that the fruits should not be eaten raw and should always be cooked until soft. Failing that, avoid them altogether.

Even forward in 1935 a health book said that a when a banana was harvested in the unripe state but subsequently appeared to be ripened, its "green starch" remained inedible. All this occurred before bananas were ripened by being artificially exposed to ethylene.

Recently, in 2007, the Chinese got into the banana fearing spirit when it was rumored that bananas from the Chinese island of Hainan could spread the SARS virus! This was the same banana that was already accused of having a hand in cancer.

As I mentioned above, by the 1940's, the banana danger movement had pretty much died down and bananas were integrated into the food culture well enough for echoes to be ignored. But there were still jokes about it, and we know that jokes after-the-fact are often the manifestations of some lingering fear.

For instance, in the 1940's, there was one about two girls on a train who bought bananas for the first time. One girl peeled her banana and took a bite. Right after she bit into the banana, the train entered a dark tunnel. "Have you started on your banana yet?" she asked the other girl. The other girl said, NO, she hadn't yet. "Well, don't…It makes you go blind."1

But then, much more recently, starting in 1999, the banana was maligned again by an online hoax. Beginning as emails, an urgent warning began to circulate on the internet, claiming to be issued by the Centers for Disease Control (CDC), although it probably started earlier via email chains. This warning said that a large shipment of bananas from Costa Rica were infected with flesh eating bacteria. The message urged everyone to avoid buying and eating bananas for several months, at the risk of contracting necrotizing fasciitis. Much later, in 2011, a similar message went out claiming the same thing about bananas from South Africa.

It is true that there was an outbreak of necrotizing fasciitis in the city of Cartago, Costa Rica in the summer of 1999, but there was never any link to bananas, or monkeys, which where also mentioned in the message. The message also instructed people, if they noticed signs of the infection, to burn their skin to halt its progress before seeing a doctor. No message from the CDC, or any medical professional would ever advise people burn their own skin!

In fact, bananas cannot be "infected" with flesh eating bacteria as "necrotizing fasciitis" is caused by bacteria, mostly streptococcus, that already live on our skin or in our bodies. To put things in perspective, these infections can be caused by the same bacteria that cause strep throat. So, as you can see, there is more to it than some scary bacteria floating around out there waiting to consume our flesh. In the United States, there are several million yearly cases of strep throat and impetigo, a skin infection caused by streptococcus. But there are only about 1000 cases of necrotizing fasciitis. Other types of bacteria are sometimes involved, as well.

Sometimes these bacteria can invade a wound (although it may rarely appear de novo) and cause a severe infection that destroys the connective tissue underneath the skin. There is no such thing as a type of bacteria that is "flesh eating bacteria." That term was invented by British tabloids in the early 1990's. These severe and spreading infections have always been around, but the problem now is antibiotic resistant bacteria. Also, many times there are predisposing factors like immuno-compromise of some kind. Snopes has a write-up of this urban myth and for more on the flesh eating bacteria scare see 50 Health Scares That Fizzled by Joan R. Callahan.

The CDC publicly announced, in both instances, that there was absolutely nothing to this. The agency had never sent out such a warning and had heard of no reports of bananas harboring flesh eating bacteria.

Why Should Unripe Bananas be Hard to Digest?

So that describes the myths, and other myths, and I think it is clear to see that it had to do with the way bananas were shipped green, to be allowed to ripen while they sat. But what of the hard to digest starch thing? Well, it is true. The starch in a green banana is, like the starch in an uncooked potato, a resistant starch. Unripe bananas, in fact, are around 82% starch and only 7% sugar. Ripe bananas are about 26% starch and 63% sugar. And overripe bananas are 3% starch and 88% sugar. Most people, once the banana is ripe, have no trouble with the starch that is left. If you tend to have a bit of GI trouble after eating bananas, you may just be particularly sensitive to the small amount of resistance starch. An allergy is also possible, of course.

banana plant with big bunch of green bananas and flower bloom

Very green bananas on a
banana plant that reminds
me a little of Seymour.

Banana ripening is actually a kind of self digestion, which is called autolysis. This involves hydrolysis of the starches, which results in the banana having more sugar content as it ages, and less starch. Digestion after consumption involves a similar process through the action of α-amylase and other enzymes.

Like potatoes and rice, the starches in bananas occur in very organized structures called grains. These are soluble in water and can resist the attack of digestive enzymes somewhat. Cooking disrupts the grains, which results in them being much more susceptible to digestion. Interestingly, once the cooked bananas cool down, the starches go back, to some extent, to their original orderly arrangement, and again become more resistant to digestion. This is called the crystallization of starch. Reheating will return them to their disrupted and more digestible arrangement.

bunch of perfectly ripe bananas, with spots

Contrary to popular belief, bananas with spots like
these are not over-ripe, they are perfect!

The same kind of thing happens when bread goes stale. This happens because the starches in the bread have crystallized. Heating up the bread again will disrupt these crystallized starches and bring the bread back, somewhat, to its former desirable state.5

Despite all this, the idea that unripe bananas cannot be digested is a stretch. They simply resist digestion, so that not all of the starch will be broken down by the digestive tracts innate action, but instead will stick around so that friendly gut bacteria will eventually break it down, through fermentation. We all know what that means!

You can eat a green banana and you may or may not get a stomach ace, depending on how sensitive you tend to be. Unless you are compromised in some way, you will certainly digest it eventually. You can also eat some small amount of raw potato, and you'll eventually digest that as well. But if you eat a large amount of either, then you can expect some problems. But who would do this?

Potato Danger!

Potatoes do deserve a special note, though! Potatoes, like tomatoes, are a member of the infamous deadly nightshade family. They contain solanine, which is a toxic substance. Now, we are often told that the toxin is only in the potato plant itself, but never in the potato. This is not actually true. In reality, there are large amounts of the toxin in the plant and only small, trace amounts, in the potato, with more being in the skin, and hardly any at all in the flesh.

Now, when the potato skins are greenish, this is a signal that there is probably more glycoalkaloids in them, which are toxins similar to that contained in the leaves. So, they pose a bit more danger. And when they are subjected to damage or diseased such as when mishandled or not stored properly, the skins produce this green tint. The green is caused by harmless chlorophyll but it is, again, a signal that there is more of the toxins being developed. An example is the potato being exposed to too much sunlight, which is why you should store your potatoes in a dark place.

baked potato with butter

The skin is the best part?

Why light? Well, when a potato is exposed to light, it produces this solanine compound as a protective chemical against being eaten by animals. Remember, the potato is a root. It's purpose is to grow a new plant and it can't very well do this if it's being scarfed up by animals like…'re not buying this are you? I thought not. The solanine compound is thought to be a defense again fungi infestation. It is not only produced upon exposure to light, but also in the areas where a potato is cut (wounded), or when subjected to blight; and as the potato ages. The chemical is an inhibitor (moderate) of acetylcholine, and this is where we encounter problems with it. But the idea that it is produced to keep us and other animals from eating the root is only speculation. Most plants have a lot more to "fear" from fungi and insects than they do from animals.3,4

potato plant held in hand with potatoes underneath

Who would fear that little guy?

Regardless, the bitter taste is there, and it is for that reason that solanine poisoning in humans is rare, except in times of food shortage. At such times people have been forced to consume blighted potatoes, and poisoning has resulted. Normally, the bitter taste should dissuade us and other animals from continued consumption after the first few bites. The upper safe level of glycoalkoloids in potatoes is generally accepted as 100 to 200mgs. Whether the toxins can build up in tissues over time, and thus result in sub-toxicity or chronic toxicity, is still being studied, but this has also been suggested to be possible.

The knowledge that potatoes were in the Nightshade family caused them to take a while to catch on with most of the world. The same thing was true of the tomato. I wrote a bit about that in my post on the history of ketchup.

Although eating large amounts of green potatoes can definitely cause solanine poisoning, you are unlikely to encounter enough of them to pose any real danger, although young children would certainly be in more danger from them. Cooking does not reliably destroy the danger, though, so if I were you I'd avoid green potatoes unless they are peeled. If you peel them, and I mean peel them well, not just the top surface of the peel, you can get rid of most all the bad stuff.6 The same thing goes for any eyes that have developed (little potato sprouts on the skin). These also will buildup the toxin and should be removed, to well underneath the skin. Most of the time, however, our potato supply is nice and of the non-green variety.

1. Fine, Gary Alan., and Bill Ellis. The Global Grapevine: Why Rumors of Terrorism, Immigration, and Trade Matter. Oxford: Oxford UP, 2010. 156-58.
2. "You Musn't Eat That." Popular Mechanics Nov. 1933: 706-07.
3. Metcalfe, Dean D., Hugh A. Sampson, and Ronald A. Simon. Food Allergy: Adverse Reactions to Food and Food Additives. Malden, MA: Blackwell Science, 2003. 404-05.
4. Schmidt, Ronald H., and Gary Eugene Rodrick. Food Safety Handbook. Hoboken, NJ: Wiley-Interscience, 2003. 227-28.
5. Brody, Tom. Nutritional Biochemistry. San Diego: Academic, 1999. 139-40.
6. Sizer, Frances Sienkiewicz., Leonard A. Piché, and Eleanor Noss. Whitney. Nutrition: Concepts and Controversies. Toronto: Nelson Education, 2012.

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